Method for determining the nh3 loading of an scr catalytic converter

ABSTRACT

A method for determining the NH 3  loading of an SCR catalytic converter in the exhaust-gas section of an internal combustion engine, in which the NH 3  concentration in the exhaust gas is determined by way of at least one sensor, preferably an NO x  sensor, downstream of the SCR catalytic converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Application of PCTInternational Application No. PCT/EP2011/071144 (filed on Nov. 28,2011), under 35 U.S.C. §371, which claims priority to Austrian PatentApplication No. A 1998/2010 (filed on Dec. 1, 2010), which are eachhereby incorporated by reference in their respective entireties

TECHNICAL FIELD

The invention relates to a method for determining the NH₃ loading of anSCR catalytic converter in the exhaust-gas section of an internalcombustion engine, wherein the NH₃ concentration in the exhaust gas isdetermined by way of at least one sensor, preferably an NO_(x) sensor,downstream of the SCR catalytic converter.

BACKGROUND

Methods for the model-based control of an SCR catalytic converter of aninternal combustion engine are known from the publications DE 10 347 130A1, DE 10 347 131 A1 and DE 10 347 132 A1, wherein the respectively useddynamic model considers the NH₃ loading of the SCR catalytic converter.A modelled NO_(x) value of the dynamic model will be adjustedcontinuously by means of an NO_(x) value measured by an NO_(x) sensorarranged downstream of the SCR catalytic converter.

Dynamic filling level models for SCR control are based on a model of theSCR catalytic converter which models the current NH₃ loading of thecatalytic converter on the basis of mass balances. As a result ofimprecisions in the sensor and actuator systems, this modelled NH₃loading can drift away from the real value during operation, which iswhy the modelled NH₃ loading needs to be adjusted to the real loading inspecific intervals.

SUMMARY

It is the object of the invention to improve the precision of fillinglevel models in a simple way.

This is achieved in accordance with the invention in such a way that thecurrent NH₃ filling level of the SCR catalytic converter is calculateddirectly via the dynamic equilibrium between NH₃ adsorption and NH₃desorption on the basis of a measured NH₃ concentration downstream ofthe SCR catalytic converter, wherein at least one physical model basedon adsorption isotherms is preferably used to determine the dynamicequilibrium.

DESCRIPTION

For the purpose of model adjustment, it is necessary at first to know ordetect by means of measuring instruments the filling level of the SCRcatalytic converter or the NH₃ concentration after the SCR catalyticconverter. This can occur in different ways, e.g. by direct measurementby means of an NH₃ sensor or by utilising the cross-sensitivity ofconventional NO_(x) sensors to NH₃— via an NO_(x) sensor. Methods areknown in literature in order to enable the reliable detection of NH₃,e.g. in propulsion operation (see DE 10 20 505 0709 A1).

If the NH₃ concentration after the real SCR catalytic converter isknown, the static NH₃ filling level in the SCR catalytic converter cansubsequently be calculated analytically via an adsorption isotherm.

It is preferably provided in this process that a Langmuir adsorptionisotherm in the form of

$\Theta_{{NH}\; 3} = \frac{K_{A} \cdot C_{{NH}\; 3}}{1 + {K_{A} \cdot C_{{NH}\; 3}}}$

is used as an adsorption isotherm, wherein Θ_(NH3) is the current NH₃loading, K_(A) the adsorption equilibrium constant and C_(NH3) theconcentration of the component NH₃ in the exhaust gas downstream of theSCR catalytic converter. The adsorption equilibrium constant K_(A) canbe determined from characteristic maps or characteristic curves. Ifparameters for the reaction kinetics of the adsorption and desorptionare known, the adsorption equilibrium constant K_(A) can also becalculated by means of the same from the equation:

$K_{A} = \frac{k_{ad} \cdot ^{- \frac{E_{ad}}{T_{c}}}}{k_{de} \cdot ^{- \frac{E_{a}}{T_{c}}}}$

wherein k_(ad) [m/s] is a pre-exponential term for the adsorption andk_(de) [m/s] a pre-exponential term for the desorption, E_(ad) [J/kmol]the activation energy for the adsorption, and E_(de) [J/kmol] theactivation energy for the desorption.

A BET isotherm in the form of:

$\Theta_{{NH}\; 3} = \frac{K \cdot q_{\max} \cdot C_{{NH}\; 3}}{\left( {C_{sat} - C_{{NH}\; 3}} \right) \cdot \left\lbrack {1 + \frac{\left( {K - 1} \right) \cdot C_{{NH}\; 3}}{C_{sat}}} \right\rbrack}$

can also be used as adsorption isotherm Θ_(NH3) as an alternative to theLangmuir adsorption isotherm, wherein Θ_(NH3) is the current NH₃loading, K the adsorption coefficient, q_(max) the maximum concentrationof the NH₃ in a layer at the surface of the exhaust gas, C_(sat) thesolubility of the NH₃, and C_(NH3) the concentration of the componentNH₃ in the exhaust gas downstream of the SCR catalytic converter.

It is further also possible to use a Freundlich isotherm in the form of:

Θ_(NH3) =K _(f) ·C _(NH3) ^(n)

as an adsorption isotherm, wherein Θ_(NH3) is the current NH₃ loading,K_(f) the Freundlich coefficient, C_(NH3) the concentration of the NH₃in the exhaust gas downstream of the SCR catalytic converter, and n theFreundlich exponent.

The described method allows a rapid and precise adjustment of the NH₃filling level of the model to the real system (static method). Robustcontrol is realised in this manner, which can achieve high NO_(x)turnovers in combination with low NH₃ slippage at the SCR catalyticconverter. This allows significantly improving the precision of dynamicfilling level models.

1-7. (canceled)
 8. A method for determining an NH₃ loading of an SCRcatalytic converter in an exhaust gas section of an internal combustionengine, comprising: measuring an NH₃ concentration in the exhaust gasdownstream of the SCR catalytic converter; and calculating a current NH₃filling level of the SCR catalytic converter via a dynamic equilibriumbetween NH₃ adsorption and NH₃ desorption on a basis of the measured NH₃concentration downstream of the SCR catalytic converter, wherein atleast one physical model based on adsorption isotherms is used todetermine the dynamic equilibrium.
 9. The method of claim 8, wherein theNH₃ concentration is determined using a sensor.
 10. The method of claim9, wherein the sensor comprises an NO_(x) sensor.
 11. The method ofclaim 8, wherein a Langmuir adsorption isotherm in the form of:$\Theta_{{NH}\; 3} = \frac{K_{A} \cdot C_{{NH}\; 3}}{1 + {K_{A} \cdot C_{{NH}\; 3}}}$is used as an adsorption isotherm, in which Θ_(NH3) is a current NH₃loading, K_(A) is an adsorption equilibrium constant, and C_(NH3) is aconcentration of the NH₃ component downstream of the SCR catalyticconverter in the exhaust gas.
 12. The method of claim 11, wherein theadsorption equilibrium constant K_(A) is determined from characteristicmaps.
 13. The method of claim 11, wherein the adsorption equilibriumconstant K_(A) is determined from characteristic curves.
 14. The methodof claim 11, wherein the adsorption equilibrium constant K_(A) iscalculated with parameters for a reaction kinetics for an adsorption anddesorption from:$K_{A} = \frac{k_{ad} \cdot ^{- \frac{E_{ad}}{T_{c}}}}{k_{de} \cdot ^{- \frac{E_{a}}{T_{c}}}}$in which k_(ad) [m/s] is a pre-exponential term for the adsorption,k_(de) [m/s] is a pre-exponential term for the desorption, E_(ad)[J/kmol] is an activation energy for the adsorption, and E_(de) [J/kmol]is an activation energy for the desorption.
 15. The method of claim 8,wherein a BET isotherm in a form of:$\Theta_{{NH}\; 3} = \frac{K \cdot q_{\max} \cdot C_{{NH}\; 3}}{\left( {C_{sat} - C_{{NH}\; 3}} \right) \cdot \left\lbrack {1 + \frac{\left( {K - 1} \right) \cdot C_{{NH}\; 3}}{C_{sat}}} \right\rbrack}$is used as an adsorption isotherm, in which Θ_(NH3) is a current NH₃loading, K is an adsorption coefficient, q_(max) is a maximumconcentration of the NH₃ in a layer at a surface of the exhaust gas,C_(sat) is a solubility of the NH₃, and C_(NH3) is a concentration ofthe NH₃ component in the exhaust gas.
 16. The method of claim 8, whereina Freundlich isotherm in a form ofΘ_(NH3) =K _(f) ·C _(NH3) ^(n) is used as an adsorption isotherm, inwhich Θ_(NH3) is a current NH₃ loading, K_(f) is a Freundlichcoefficient, C_(NH3) is a concentration of the NH₃ in the exhaust gasdownstream of the SCR catalytic converter, and n is a Freundlichexponent.
 17. The method of claim 16, further comprising adjusting adynamic NH₃ filling level model to a static NH₃ loading.
 18. A methodfor determining an NH₃ loading of an SCR catalytic converter in anexhaust gas section of an internal combustion engine, comprising:measuring an NH₃ concentration in the exhaust gas downstream of the SCRcatalytic converter; calculating a current NH₃ filling level of the SCRcatalytic converter via a dynamic equilibrium between NH₃ adsorption andNH₃ desorption on a basis of the measured NH₃ concentration downstreamof the SCR catalytic converter; and adjusting a dynamic NH₃ fillinglevel model to a static NH₃ loading, wherein at least one physical modelbased on adsorption isotherms is used to determine the dynamicequilibrium.
 19. The method of claim 18, wherein the NH₃ concentrationis determined using a sensor.
 20. The method of claim 19, wherein thesensor comprises an NO_(x) sensor.
 21. The method of claim 18, wherein aLangmuir adsorption isotherm in the form of:$\Theta_{{NH}\; 3} = \frac{K_{A} \cdot C_{{NH}\; 3}}{1 + {K_{A} \cdot C_{{NH}\; 3}}}$is used as an adsorption isotherm, in which Θ_(NH3) is a current NH₃loading, K_(A) is an adsorption equilibrium constant, and C_(NH3) is aconcentration of the NH₃ component downstream of the SCR catalyticconverter in the exhaust gas.
 22. The method of claim 21, wherein theadsorption equilibrium constant K_(A) is determined from characteristicmaps.
 23. The method of claim 21, wherein the adsorption equilibriumconstant K_(A) is determined from characteristic curves.
 24. The methodof claim 21, wherein the adsorption equilibrium constant K_(A) iscalculated with parameters for a reaction kinetics for an adsorption anddesorption from:$K_{A} = \frac{k_{ad} \cdot ^{- \frac{E_{ad}}{T_{c}}}}{k_{de} \cdot ^{- \frac{E_{a}}{T_{c}}}}$in which k_(ad) [m/s] is a pre-exponential term for the adsorption,k_(de) [m/s] is a pre-exponential term for the desorption, E_(ad)[J/kmol] is an activation energy for the adsorption, and E_(de) [J/kmol]is an activation energy for the desorption.
 25. The method of claim 18,wherein a BET isotherm in a form of:$\Theta_{{NH}\; 3} = \frac{K \cdot q_{\max} \cdot C_{{NH}\; 3}}{\left( {C_{sat} - C_{{NH}\; 3}} \right) \cdot \left\lbrack {1 + \frac{\left( {K - 1} \right) \cdot C_{{NH}\; 3}}{C_{sat}}} \right\rbrack}$is used as an adsorption isotherm, in which Θ_(NH3) is a current NH₃loading, K is an adsorption coefficient, q_(max) is a maximumconcentration of the NH₃ in a layer at a surface of the exhaust gas,C_(sat) is a solubility of the NH₃, and C_(NH3) is a concentration ofthe NH₃ component in the exhaust gas.
 26. The method of claim 18,wherein a Freundlich isotherm in a form ofΘ_(NH3) =K _(f) ·C _(NH3) ^(n) is used as an adsorption isotherm, inwhich Θ_(NH3) is a current NH₃ loading, K_(f) is a Freundlichcoefficient, C_(NH3) is a concentration of the NH₃ in the exhaust gasdownstream of the SCR catalytic converter, and n is a Freundlichexponent.